Definition - What does Unique Constraint mean?
A unique constraint is a type of column restriction within a table, which dictates that all values in that column must be unique though may be null.
To ensure that a column is UNIQUE and cannot contain null values, the column must be specified as NOT NULL. Interestingly, these are a primary key’s two main attributes. Defining both attributes in a newly-created column should be given serious consideration for the primary key designation.
Techopedia explains Unique Constraint
A unique constraint is defined at the time a table is created. A unique constraint allows null values. Initially, this may seem like a contradiction, but a null is the complete absence of a value (not a zero or space). Thus, it is not possible to say that the value in that null field is not unique, as nothing is stored in that field. A null value cannot be compared to an actual value. For example, the Queen of America cannot be compared to the Queen of England because the Queen of America is a null that does not exist.